The Kruger area tends to be warmer than if you head further south, so has year round warm weather although during winter the early mornings and evenings can be chilly, so a fleece or jacket is a good option when packing for game drives.
Jun-August are classed as the winter months and are also the driest. This can be good for safari as the animals look for available water sources, making them easier to spot.
Summer months are hot and humid and there can be heavy rainfall at times, so expect this if you are planning to travel Dec through to February.
The in between months are often a good time to travel in terms of finding balance with the weather conditions, although you will find game abundant year round.